1. Field of the Invention
This invention relates to a method for treating aerosol smokes or fogs to remove the suspended solid or liquid particles from the suspending gaseous phase. This invention is useful in providing a method for treating emissions from a power plant or a smelter, wherein said emissions comprise positively charged particles which are difficult to remove with liquid scrubbers and therefore require electrostatic precipitators--a capital intensive addition to any process for treating the emissions from a power plant or a smelter.
2. Summary of the Art
Suspensions of solid or liquid particles in a gas are known generally as aerosols. In particular, liquid particles formed by condensation or dispersion of a gas are known as aerosol fogs. Solid particles suspended in a gaseous phase are known as aerosol smokes. Aerosol smokes are also usually the result of the condensation of solid particles in a gaseous phase. Aerosols which are formed by condensation process may include very small particles, i.e., less than about 0.1 micron. These particles are generally electrostatically charged and, therefore, due to the size and the electrostatic charge of such particles, it is quite difficult to treat an aerosol to separate the suspended phase from the gaseous phase.
In view of the above, some of the most intractable pollution emission problems are associated with aerosol smokes and fogs. It is frequently impossible to remove the suspended particles from aerosols with conventional cyclones and scrubbing equipment. It is known that aerosol smokes and fogs may be treated to flocculate the suspended particles through the use of high voltage electrostatic generators which effectively neutralize the charged particles on a charged surface. The flocculated particles, having an increased size, are more easily removed from the gaseous phase than non-flocculated aerosol particles. It is noted, however, that electrostatic precipitators are the most capital intensive of all the control devices for pollution. Also, mechanical, electrical, or process problems can cause poor on-stream time and reliability of such electrostatic precipitators. For example, although electrostatic precipitators are available which either neutralize positive or negative charges, negative polarity discharge electrodes are practically always utilized because high voltages can be obtained without spark over. If a positive polarity discharge electrode must be used, a two stage unit is required.
Another problem with electrostatic precipitators, especially when such precipitator is utilized to treat an aerosol smoke, is that the collected solid particles adhere to the collecting surface. Thus, the collecting surface must be rapped at certain predetermined intervals and intensities to remove the collected dust. This rapping causes further problems since the collected dust may be reentrained in the gas stream. The resistivity of some dusts may also cause electrical problems of arcing and control. Moreover, an aerosol smoke cannot be passed through the electrostatic precipitator at a high velocity without efficiency loss since a high velocity may also cause reentrainment of the collected dust into the moving aerosol smoke. Thus, in designing a process for electrostatic precipitation of an aerosol smoke, an optimum time interval between rapping to remove collected dust, an optimum rapping force and an optimum flow rate must be determined.
The resistivity of the suspended solid phase of the aerosol also is important to an efficient removal process. If the resistivity is too low, there may be a drop in field potential across the collected particulate layer. Also, particulates having a low resistivity may lose their charge to the collecting plate and tumble off randomly and be reentrained in the gaseous phase. However, when the particulate is very conductive, electrostatic precipitators are generally unsatisfactory collection devices. With aerosols having very conductive particles, the precipitator is used only to agglomerate the particles, with the agglomerated particles being trapped downstream from the electrostatic precipitator in a cyclone. This, of course, requires an additional capital investment.
An improvement in the use of electrostatic precipitators to remove suspended particles from aerosols has been made recently. These improved electrostatic precipitators use water sprays to solve some of the conductivity problems noted above. In one variation of this improved process, the water droplets have an impressed charge to assist in collecting the suspended particles. It has been stated that the combination of the water spray and electrostatic precipitation enhances the collection process but there are problems as well as advantages. For example, water can enhance latent corrosion problems and require the use of expensive alloys where carbon steel might be satisfactory in a dry electrostatic precipitator. Moreover, the water utilized must be treated and can cause waste-handling problems. For example, recirculated spray water can be supersaturated with low solubility compounds that build up when the water is returned to the electrostatic precipitator. Suspended solids in the recirculated spray water can also erode or plug nozzles. In view of this, "wet" precipitators have been used generally to date for treating mixtures of gases and submicron particles such as aluminum potline and carbon anode baking fumes, fiberglass fume control, coke oven and metallurgical fumes and phosphate emissions.
It is clear from the above, that an improved method for treating aerosol smokes and fogs to remove the suspended particulate phase from the gaseous phase would be desirable. Therefore, it is one object of this invention to provide a method for treating an aerosol smoke or fog to remove suspended particulate matter from the gaseous phase, which method does not include electrostatic precipitation, alone, or in the presence of a water spray.
It is another object of the instant invention to provide a method for treating the emissions from a power plant or smelter wherein the emission comprises an aerosol smoke having positively charged particles.
Additional objects, advantage, and features of the invention will be come apparent to those skilled in the art from the following description.